In the past decade the use of metallic or zero-valent iron (ZVI) in the form of iron fillings or powders for the reduction of halogenated organic contaminants such carbon tetrachloride (CT, CCL4), chloroform (CF, CHCl3), trichloroethylene (TCE, C2HCl3), and tetrachloroethene (PCE, C2Cl4) for environmental remediation has emerged. Iron is a moderate reducing reagent. Reactions of iron with dissolved oxygen and to some extent with water are the main reactions in classical electrochemistry (e.g., corrosion). The corrosion reactions can be inhibited or accelerated by manipulating solution chemistry and/or solid (metal) composition. This is echoed in the transformation of hazardous and toxic chemicals in which iron oxidation is coupled to the contaminant reduction. For example, tetrachloroethene (C2Cl4), a common solvent, can accept electrons from iron oxidation and be reduced to ethane in accordance with the following reaction:C2Cl4+4Fe0+4H+→C2H4+4Fe2+4Cl−
Environmental applications of metallic iron have been enthusiastically accepted by many users and regulatory agencies, largely due to the low cost and absence of any known toxicity induced by iron.
So far, the main approach for the application of zero-valent iron in environmental remediation is to build in-ground barriers filled with zero-valent iron. Barriers containing zero-valent iron are typically installed across the flow path of groundwater. When the contaminated water passes through the reactive or adsorptive materials, contaminants in the water are removed by various physical and chemical mechanisms. Such treatment barriers are often called “permeable reactive barriers” (PRBs) as the barriers are usually more water permeable than the native soil/sediment materials. Examples of such techniques are remediants are disclosed in U.S. Pat. Nos. 5,624,552; 5,759,389; 6,242,663 B1; and 6,287,472 B1 and U.S. Patent Application Publication No. US 2003/0134409 A1.
It should be noted that permeable reactive barriers are often installed in the downstream direction of the contaminated plume flow or development to contain the spread of the contaminant plume. This represents a passive approach for site remediation as the contaminant source(s) can not be treated directly. Iron is heavy (7,800 kg/m.sup.3) so that large amounts of iron are needed to construct an iron permeable reactive barrier. Furthermore, construction cost is relatively high, especially for deep aquifers. Construction of such barriers at many sites might not be feasible due to the presence of existing high value structures such as buildings and airport runways. Those shortcomings may have significantly limited the applications of iron permeable reactive barriers.
One solution to the problem of using zero-valent iron particles to remediate ground and groundwater contaminants is set forth in U.S. Pat. No. 7,128,841, the specification of which is incorporated herein by reference.